The development of the tetracycline antibiotics was the direct result of a systematic screening of soil specimens collected from many parts of the world for evidence of microorganisms capable of producing bacteriocidal and/or bacteriostatic compositions. The first of these novel compounds was introduced in 1948 under the name chlortetracycline. Two years later oxytetracycline became available. The detailed elucidation of the chemical structure of these agents confirmed their similarity and furnished the analytical basis for the production of a third member of this group in 1952, tetracycline. By 1957, a new family of tetracycline compositions characterized chemically by the absence of the ring-attached CH.sub.3 group present in the earlier compositions was prepared and became publically available in 1959 under the official name demeclocycline. Subsequently, methacycline, a derivative of oxytetracycline, was introduced in 1966; doxycycline became available by 1967; and minocycline was in use by 1972. For clarity, for general ease of understanding, and for comparison purposes, these individual tetracycline type agents are structurally compared within Table I below.
TABLE I ______________________________________ TETRACYCLINE ##STR1## At Carbon Congener Substituent(s) Position Nos. ______________________________________ Chlorotetracycline Cl (7) Oxytetracycline OH, H (5) Demeclocycline OH, H; Cl (6;7) Methacycline OH, H; CH.sub.2 (5;6) Doxycycline OH, H; CH.sub.3, H (5;6) Minocycline H, H; N(CH.sub.3).sub.2 (6;7) ______________________________________
Subsequent to these initial developments, much research effort was focused on developing new tetracycline antibiotic compositions effective under varying therapeutic conditions and routes of administration; and for developing new tetracycline analogues which might prove to be equal or more effective than the originally introduced tetracycline families beginning in 1948. Representative of such developments are U.S. Pat. Nos. 3,957,980; 3,674,859; 2,980,584; 2,990,331; 3,062,717; 3,557,280; 4,018,889; 4,024,272; 4,126,680; 3,454,697; and 3,165,531. It will be understood that these issued patents are merely representative of the range of diversity of investigations seeking tetracycline and tetracycline analogue compositions which are pharmacologically active.
Historically, soon after their initial development and introduction, the tetracyclines regardless of specific formulation or chemical structure were found to be highly effective pharmacologically against rickettsiae; a number of gram-positive and gram-negative bacteria; and the agents responsible for lymphogranuloma venereum, inclusion conjunctivitis, and psittacosis. Hence, tetracyclines became known as "broad spectrum" antibiotics. With the subsequent establishment of their in-vitro antimicrobial activity, effectiveness in experimental infections, and pharmacological properties, the tetracyclines as a class rapidly became widely used for therapeutic purposes. However, this widespread use of tetracyclines for both major and minor illnesses and diseases led directly to the emergence of resistance to these antibiotics even among highly susceptible bacterial species both commensal and pathogenic--as for example Pneumococci and Salmonella. The rise of tetracycline-resistant organisms has led not only to a general decline and avoidance of tetracyclines and tetracycline analogue compositions as antibiotics of choice but has also launched major efforts and investigations to uncover the mechanism for tetracycline resistance--in the hope that some effective means might be developed to overcome the problem of tetracycline-resistance and thus reestablish the pharmacological value and efficacy of tetracyclines as a whole.
The following represents a current summary of the investigations and knowledge regarding the mechanism of action for tetracyclines in bacteria. The site of action for tetracyclines is the bacterial ribosome; and that at least two different processes appear to be required for tetracyclines to gain access to the cytoplasm and the ribosomes of bacteria. The first process is a passive diffusion of the tetracycline through hydrophilic pores located in the outer cell membrane. One of these structures is the major outer membrane protein, Omp F. The second process involves an energy-dependent active transport system that pumps all tetracyclines through the inner cytoplasmic membrane. In the tetracyline-sensitive cell or organism, once the tetracycline gains access to the interior of the cell, it is able to bind to the ribosomes and to inhibit protein synthesis. However, in many tetracycline resistant cells and organisms, an efflux pump system is present which appears to bind the tetracycline molecule and actively transports the tetracycline molecule out of the organism into the surrounding environment. This active efflux employs an inner membrane protein designated TET (or Tet) protein which is synthesized in the cell from a gene which is generally acquired by the organism. Often the gene is present on an extra-chromosomal, autonomously replicating plasmid or a transposon.
Tetracycline resistance is often regulated--that is, inducible by tetracycline. Investigations of active tetracycline efflux systems and the details of the active efflux mechanism of action have been well documented and include the following publications, each of which is expressly incorporated by reference herein: Chopra et al., J. Antimicrobiol. Chemotherapy 8:5-21 (1981); Levy and McMurry, Biochem. Biophys. Res. Comm. 56:1060-1068 (1974); Levy and McMurry, Nature 275:90-92 (1978); McMurry and Levy, Antimicrobial Agents And Chemotherapy 114:201-209 (1978); McMurry et al., Proc. Nat. Acad Sci. U.S.A. 77:3974-3977 (1980); Ball et al., Biochem. Biophys. Res. Comm. 93:74-81 (1980); Curiale and Levy, J. Bact. 151:209-2115 (1982); Mendez et al., Plasmid 3:99-108 (1980); Curiale et al., J. Bact. 157:211-217 (1984); and Levy, S. B., Journal Of Antimicrobial Chemotherapy 24:1-3 (1989).
In addition, a second mechanism of tetracycline resistance for cells is known and in effect. This resistance mechanism involves a cytoplasmic protein which protects the intracellular ribosomes from the inhibitory action of tetracyclines. This form of tetracycline resistance is described within Burdett, V., J. Bact. 165:564-569 (1986); and Levy, S. B., J. Antimicrob. Chem. 24:1-3 (1989).
With the increased understanding and knowledge regarding the origin and the mechanisms of tetracycline resistance in various cells and microorganisms, overt investigations and developments seeking means for overcoming these mechanisms, notably the active efflux system have been attempted. One successful approach is described within U.S. Pat. No. 4,806,529 issued Feb. 21, 1989--an innovation which is a precursor of more recent developments. Clearly, additional methods and materials for overcoming tetracycline-resistance in bacteria and other organisms is most desirable and needed. Substantive advances which additionally overcome the active efflux system for tetracycline in the resistant cell would be presently recognized by the ordinary practitioner in the art as a major asset and innovation.